Arsenic is a naturally occurring metalloid and widespread environmental contaminant, primarily presenting a public health challenge through contaminated drinking water. Because it is odorless and tasteless, its presence is undetectable without specific testing. The danger associated with arsenic exposure is not typically from acute, high-dose poisoning, but from chronic ingestion of low concentrations over many years. Long-term, cumulative exposure to the inorganic form of arsenic poses the most significant health threat.
Establishing the Safety Threshold
The level of arsenic in water considered safe is defined by regulatory standards focused on minimizing the risk from lifetime exposure. The U.S. Environmental Protection Agency (EPA) sets the enforceable limit for public water systems under the Safe Drinking Water Act (SDWA). This standard, known as the Maximum Contaminant Level (MCL), is currently 0.010 milligrams per liter (mg/L), equivalent to 10 parts per billion (ppb).
The 10 ppb standard was established to protect consumers from the harmful effects of long-term ingestion. This limit replaced an older standard of 50 ppb after scientific evidence demonstrated health risks at lower concentrations. The World Health Organization (WHO) also recommends a guideline of 10 ppb for arsenic in drinking water.
While the MCL of 10 ppb is the legally enforced limit, the EPA also sets a non-enforceable health goal called the Maximum Contaminant Level Goal (MCLG). For arsenic, the MCLG is set at zero, reflecting the scientific understanding that arsenic is a known human carcinogen. For substances that cause cancer, no level of exposure is considered completely risk-free. The difference between the zero goal and the 10 ppb limit accounts for the practical and economic challenges of water treatment technology.
Health Risks Associated with Exposure
Long-term consumption of water with arsenic concentrations above the safety threshold is associated with serious chronic health effects. Arsenic is classified as a human carcinogen, and long-term exposure increases the risk of several cancers, including those of the bladder, lung, and skin. Evidence also links chronic exposure to cancers of the kidney, liver, and prostate.
Beyond cancer, chronic exposure can lead to distinctive dermatological issues, often the first noticeable signs of arsenic poisoning. These include hyperpigmentation, characterized by diffuse darkening or “raindrop-like” spots on the skin, and keratosis, a thickening of the skin, particularly on the palms and soles. These skin lesions can take a minimum of five years to develop.
The metalloid also affects multiple other organ systems, contributing to cardiovascular problems such as heart disease and hypertension. Neurological effects, including peripheral neuropathy, can manifest as numbness and muscle weakness. Studies have also linked chronic low-level exposure to conditions like Type 2 diabetes and negative impacts on cognitive development, especially in children.
Acute exposure, typically occurring at very high doses (often exceeding 130 ppb), can cause severe gastrointestinal distress, including vomiting and diarrhea, leading to dehydration and cardiovascular collapse. However, the primary health concern is low-level chronic exposure, which slowly increases the risk of disease over a lifetime. Research suggests that even at levels below the 10 ppb MCL, such as in the 5 to 10 ppb range, there is an increased risk of cardiovascular disease.
Sources of Contamination and Testing Methods
The primary source of arsenic contamination in drinking water is natural, geological processes. Arsenic is a component of the earth’s crust, and as groundwater moves through rock formations and soil, it dissolves naturally occurring arsenic, carrying it into aquifers and wells. Contamination is often localized to specific geographic regions where the underlying geology is prone to releasing arsenic.
While natural deposits are the main cause, human activities also contribute to contamination through industrial and agricultural runoff. Past uses of arsenic in pesticides, wood preservatives, and mining operations have left residues that can leach into groundwater supplies. The risk is higher in groundwater sources compared to surface water.
Public water systems are required to regularly test for arsenic and report the results to customers annually. However, owners of private wells are solely responsible for ensuring the safety of their water supply, as private wells are not subject to EPA regulations. Testing is the only way to confirm if arsenic is present.
Private well owners should have their water tested by a state-certified laboratory. It is recommended to test at least once a year, or twice annually to account for seasonal variations. The test must be specific for arsenic, and results should be compared directly to the 10 ppb MCL. If the result exceeds this level, the water should be treated or an alternative source used for drinking and cooking.
Water Treatment and Mitigation Strategies
If testing reveals arsenic levels above 10 ppb, implementing a water treatment system is the most effective mitigation strategy. The choice depends on whether treatment is needed for the entire house (point-of-entry or POE system) or only for a single tap used for drinking and cooking (point-of-use or POU system). POU systems are often a practical and less expensive option, treating only the small volume of water consumed.
Highly effective treatment methods for arsenic include reverse osmosis (RO), adsorption media, and anion exchange.
Reverse Osmosis (RO)
Reverse osmosis (RO) systems use a semipermeable membrane to filter out contaminants, including both forms of inorganic arsenic: arsenite (As(III)) and arsenate (As(V)).
Adsorption Media
Adsorption media filters, particularly those using iron oxide or activated alumina, bind the arsenic to the media’s surface, removing it from the water.
Anion Exchange and Oxidation
Anion exchange systems are also effective, but they primarily remove arsenate (As(V)). Because arsenite (As(III)) is more difficult to remove, many treatment methods are more efficient when the arsenite is first oxidized, or converted, into arsenate.
Maintenance and Ineffective Methods
Common household filters, such as standard activated carbon filters used in pitcher filters, are generally ineffective for arsenic removal. All effective treatment systems require regular maintenance, including the replacement of filter media or membranes, to maintain their effectiveness over time.